1. Field of the Invention
This invention relates to a lubricating oil composition, and specifically to a lubricating oil composition which is excellent in detergency, NO.sub.x oxidation resistance and thermal oxidation resistance and is suitable as a long-life engine oil for gas engine heat pumps.
2. Prior Art
Concerning gas engine heat pumps (hereinafter abbreviated as "GHP"), research and development work have been started in recent years as a part of the gas-powered air-conditioning popularization promoting policy. This research and development work has already led to commercialization of gas engine heat pump air conditioners and the like. As the popularization of these apparatuses has proceeded further, an increasing need has, however, arisen for their maintenance and inspection work. This has led to an important theme, that is, a need for improvements in maintenance such as simplification of an inspection and prolongation of maintenance work intervals. In particular, prolongation of a drain interval of an engine oil has become the key to improvements in maintenance.
On the other hand, a GHP engine oil is accompanied by the problem that it is prone to extremely premature deterioration upon contact with NO.sub.x contained at a high concentration in blowby gas because of the structure of a GHP apparatus and a high combustion temperature. As quality requirements for the GHP engine oil, the following properties are therefore required especially:
(1) excellent anti-NO.sub.x performance, PA1 (2) excellent thermal oxidation resistance, and PA1 (3) dispersibility of residues in oil. PA1 (A) 0.5 wt % to 10 wt % of a metal salicylate having a total base number (TBN) of from 100 mg-KOH/g to 195 mg-KOH/g; PA1 (B) 0.1 wt % to 10 wt % of at least one oxidation inhibitor selected from the group consisting of amine compounds; PA1 (C) 0.1 wt % to 10 wt % of at least one oxidation inhibitor selected from the group consisting of hindered phenol compounds; and PA1 (D) 1 wt % to 10 wt % of a polyalkenylsuccinimide and/or a boron-containing polyalkenylsuccinimide. PA1 (i) a lubricating oil composition comprising a lubricating base oil and based on the whole weight of the lubricating oil composition, PA1 (ii) More preferably, the lubricating oil composition comprises a mineral base oil and/or a synthetic base oil and based on the whole weight of the lubricating oil composition: PA1 (iii) Another preferred embodiment comprises a lubricating oil composition comprising a mineral base oil and/or a synthetic base oil and based on the whole weight of the lubricating oil composition: PA1 at least one group or atom selected from the group consisting of: ##STR18## Exemplary examples of the hydrocarbon group can include alkyl groups having 1-40 carbon atoms, alkenyl groups having 2-40 carbon atoms, cycloalkyl groups having 3-40 carbon atoms, aryl groups having 6-40 carbon atoms, alkylaryl groups having 7-40 carbon atoms, arylalkyl groups having 7-40 carbon atoms, and the like. PA1 at least one group or atom selected from the group consisting of: ##STR19## In the formula, X is --S-- or a hydrocarbon group having 1-45 carbon atoms, and the hydrocarbon group may be linear, branched, cyclic or aromatic and may contain one or more double bond. Further, the hydrocarbon group may contain in the structure thereof: PA1 at least one group or atom selected from the group consisting of: ##STR20## PA1 (A) 1 wt % to 8 wt % of an alkaline earth metal salicylate having a total base number of from 150 mg-KOH/g to 190 mg-KOH/g; PA1 (B) 0.5 wt % to 5 wt % of a dialkyldiphenylamine and phenyl-.alpha.-naphthylamine; PA1 (C) 0.5 wt % to 5 wt % of 2,2-thio[diethyl bis3(3,5-di-t-butyl-4-hydroxyphenol)propionate] and 4,4'-methylenebis(2,6-di-t-butylphenol); PA1 (D) 2 wt % to 8 wt % of a boron-containing polyalkenylsuccinimide; and PA1 (E) 0.3 wt % to 4 wt % of an alkaline earth metal phenate;
Nonetheless, GHP engine oils which have been proposed to date cannot meet all of the above-mentioned quality requirements, but are still insufficient not only in NO.sub.x oxidation resistance but also in thermal oxidation resistance. They are also prone to form sludge and deposits due to NO.sub.x deterioration. NO.sub.x deterioration is estimated to take place in such a way that NO.sub.x would attack a base oil and additives in an engine oil to form highly reactive radicals and deterioration would then proceed due to NO.sub.x, oxygen and heat. The sludge so formed contains reaction products of the base oil and NO.sub.x (RONO.sub.2, R: hydrocarbon groups), reaction products of the additives and NO.sub.x, oxidation-deteriorated products of the base oil (RCOOH, R: hydrocarbon groups), and engine oil components. Their presence brings about an increase in viscosity, an increase in acid number and the like for the engine oil, whereby lubricating performance is significantly impaired. For a GHP oil susceptible to such influence, a high degree of detergency is hence required.
With the foregoing circumstances in view, the present invention therefore has as an object thereof the provision of a lubricating oil composition which is excellent in all the properties of detergency, NO.sub.x oxidation resistance and thermal oxidation resistance and is suitable as a long-life GHP engine oil.